that fly machine is probly only working your rear delts, save that for shoulder day.
(bent over rows)I usually stand about 45 degree angle, something like that, not 90. As long as you feel it in your back more than your arms your doin it right.
I find seated rows to target thickness in my middle back the best. Close grip. And make sure to get a full stretch, lessn up the weight if you have to. I see guys bottom pin that machine then hug the cable to their chest only moving the bar a few inches or half a foot. I get as much of a stretch as possible, its definitly helped me over the keeping your back stiff version.